Very rarely a defective metal halide lamp will blacken to the extent that a continuous film of tungsten is deposited over the inner arc tube surface, which forms a short circuit across the electrodes. If all conditions are favourable it then turns into an incandescent lamp and the coated arc tube starts glowing. That might be what is happening here. It is rare to have a sodium-rich metal halide lamp, because usually sodium is the first metal to disappear during life and it is usually mercury and thallium that are left behind, giving a blue-green colour.

Very rarely a defective metal halide lamp will blacken to the extent that a continuous film of tungsten is deposited over the inner arc tube surface, which forms a short circuit across the electrodes. If all conditions are favourable it then turns into an incandescent lamp and the coated arc tube starts glowing.